The U.S. government’s only facility for handling, processing and storing weapons-grade uranium was temporarily shut this week after anti-nuclear activists, including an 82-year-old nun, breached security fences, government officials said on Thursday.

WSI Oak Ridge, the contractor responsible for protecting the facility at Oak Ridge, Tennessee, is owned by the international security firm G4S, which was at the center of a dispute over security at the London Olympic Games.

Officials said that the facility was shut down on Wednesday at least until next week after three activists cut through perimeter fences to reach the outer wall of a building where highly-enriched uranium, a key nuclear bomb component, is stored.

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The activists painted slogans and threw what they said was human blood on the wall of the facility, one of numerous buildings in the facility known by the code name Y-12 that it was given during World War II, officials said.

While moving between the perimeter fences, the activists triggered sensors which alerted security personnel. However, officials conceded that the intruders still were able to reach the building’s walls before security personnel got to them.

It is unbelievable this could happen. The significance is outrageous. If they were terrorists, they could have blown open the door and got inside

Ellen Barfield, a spokeswoman for the activists who called themselves “Transform Now Plowshares”, were arrested and charged by federal authorities with vandalism and criminal trespass.

She said the three, identified as Megan Rice, 82, Michael R. Walli, 63 and Greg Boertje-Obed, 57, were being held in custody. They are scheduled to appear in court in the next few days.

Barfield forwarded a statement from the group in which they said they had passed through four fences and walked for “over two hours” before reaching the uranium storage building, upon which they hung banners and strung crime-scene tape.

Officials said that the storage building itself, which was built after the September 11, 2001 al Qaeda attacks on New York and Washington, was designed with extra and modern security features and that its contents were not compromised.

WSI Oak Ridge, the private firm employed by the U.S. Department of Energy to provide security at Y-12, is a subsidiary of the giant international security firm G4S.

G4S drew sharp criticism for failing to provide the number of security personnel it promised to protect the London Olympic Games, forcing the British government to deploy extra army troops.

A spokeswoman for G4S declined to comment and referred inquiries to government spokespeople.

We’re taking this very, very seriously

The security failure was an embarrassment both for the security firm and for the National Nuclear Security Administration (NNSA), the Energy Department branch which operates U.S. nuclear weapons plants. “It was obviously a pretty serious incident,” NNSA spokesman Joshua McConaha told Reuters.

“We’re taking this very, very seriously,” added Steve Wyatt, a spokesman for the NNSA office in Oak Ridge which supervises the activities of Y-12 contractors.

The NNSA officials said that the activists cut through two chain link fences surrounding the sprawling facility and a third fence surrounding the ultra-secure enriched uranium stockpile building, known as the “Highly Enriched Uranium Materials Facility.”

Wyatt said that the building serves as the U.S. government’s only “warehouse” for storing highly enriched uranium used in nuclear weapons.

Highly enriched uranium is a radioactive material used in the core of bombs to produce a nuclear detonation. The Oak Ridge plant is one of the most important government installations involved in the maintenance and production of the U.S. nuclear arsenal.

Although the security breach occurred overnight last Friday, officials confirmed that the shutdown – which applies to “all nuclear operations” at the Y-12 site – did not begin until Wednesday. Officials said that it was expected to continue into next week.

In the meantime personnel at the facility would be given additional security training.

Peter Stockton, a former Congressional investigator and security consultant to the Energy Department, expressed skepticism at government claims that the nuclear material was not at risk.

“It is unbelievable this could happen,” Stockton said. “The significance is outrageous. If they were terrorists, they could have blown open the door and got inside.”

Stockton said that the security breach was the “worst we’ve ever seen.” He said it was more serious than the case of Wen Ho Lee, a Taiwan-born scientist who was suspected of espionage at the Los Alamos nuclear laboratory. He pleaded guilty in 2000 to a less severe charge when the case against him collapsed.